Oyster Creek Generating Station was shut down for maintenance last week. Before power was restored Saturday night, officials found about 50 fish, floating near a discharge canal, dead.
Now, plant officials and environmental experts are trying to figure out what killed the fish.
"We can't call it a fish kill at this point, because we don't know if the problem was plant-related," said plant spokesman David Benson.
Benson said Exelon Nuclear, which owns and operates the plant, hired an independent environmental contractor to analyze the fish. There's no word on how long that process will take. The plant also notified the Department of Environmental Protection.
Benson said water temperatures in the discharge canal have been stable, changing only with natural environmental conditions.